The Role of Solvent in Carbon Quantum Dot Synthesis on the Performance of MoS₂ Nanosheet/Carbon Quantum Dot Heterostructures as Electrocatalysts for the Hydrogen Evolution Reaction

This study investigates the effect of different solvents used in the synthesis of carbon quantum dots (CQDs) on the electrocatalytic performance of MoS2/CQD heterostructures for the hydrogen evolution reaction (HER). While previous research focused on CQDs synthesized with deionized water, little attention has been given to the influence of other solvents on CQD electrocatalytic behavior. To address this, we synthesized MoS2 on 3D carbon cloths via a hydrothermal method and subsequently incorporated CQDs synthesized using deionized water, glycerol, and dimethylformamide (DMF). The choice of solvent significantly impacts their morphology, crystallinity, surface, and electrochemical properties. In particular, MoS2 nanosheets became smaller with increased disordered structures and defect sites, particularly sulfur vacancies. Among the heterostructures, MoS2/CQDs-Glycerol showed superior performance, with an onset overpotential of 130 mV and Tafel slope of 53 mV/dec at 10 mA/cm2, outperforming MoS2/CQDs-DI (149 mV, 68 mV/dec) and MoS2/CQDs-DMF (185 mV, 106 mV/dec). The enhanced performance of MoS2/CQDs-Glycerol is attributed to its larger active surface area (Cdl of 228.7 mF/cm2) and lower charge transfer resistance (Rct of 2.25 Ω), which may be due to the formation of more Mo–S edges on the vertical plane, serving as active sites. This study demonstrates that glycerol is the most effective solvent in CQD synthesis for enhancing HER performance by improving the morphology, surface properties, and charge transfer.